Motor graders have a longitudinal main frame which has a dirigible wheel assembly at its forward end, an operator's cab at its rearward end portion, and a traction chassis for the motor and power train behind the cab. The motor grader blade is suspended from the main frame by means of a circle draw bar and a circle. The circle draw bar has its front end connected to the front of the main frame by a ball and socket connection, while the rearward portion of the circle draw bar is suspended from the main frame by hydraulic cylinder and piston means which permit the draw bar to swing in a vertical plane about its front end.
The circle is mounted on the rearward portion of the circle draw bar for rotation about a vertical axis, and there is a driving interconnection between a motor on the circle draw bar and a ring gear on the circle to effect such rotary motion of the circle.
The grader blade is mounted upon the circle so that rotation of the circle changes the angle of the blade with reference to the path of travel of the grader, while swinging the circle draw bar in a vertical plane about its forward end changes the vertical position of the grader blade with reference to the ground.
In addition, the grader blade is mounted on a horizontal axis so that it may be tipped with respect to the circle by hydraulic cylinder and piston means to change the angle of attack of the blade.
Different types of circle draw bar and circle structures are illustrated in U.S. Pat. Nos. 2,497,778, 3,421,589, and 3,470,967. A type of grader structure in which the grader is towed behind a tractor instead of being at the front of a long grader vehicle, but which has a similar grader blade mounting, is illustrated in U.S. Pat. No. 2,498,044.
Typical grading operations place enormous stresses upon the circle draw bar, the circle and related parts of a motor grader. In operation the grader blade produces both vertical and lateral stresses in the entire system, and the direction and magnitude of those stresses varies depending upon the particular type of work being performed. Accordingly, it is difficult to engineer a grader blade mounting system which has adequate stress resistance without using excessively heavy components that increase grader cost and energy requirements.